I'm not talking about Shane killing the zombies as his 'wrong', although how he did it was pretty stupid, since it put everyone at risk including Carl.
I'm talking about Shane taking the farm by force from Herschel, which is his point - killing the zombies was just an means to an end. When he left Lori, he left intending to take the farm. That's why he told Carl they would do whatever it took to stay, and that's why he went to the RV looking for the guns. He has no intention of stopping with clearing the barn (cm'on, do you think Shane is stupid enough to think Herschel is going to say "Oh, thanks so much for killng those zombies, please stay with us" now?), he intends on taking the farm for theirs no matter what that means, and by clearing the barn he's forced the situation. That's what I'm talking about as being the 'wrong' Shane has justified in his own mind.
Funny thing is, he's actually doomed Lori far worse by being rash and emotional. Lori needed Herschel more than she needs his farm - when it comes time to deliver, a doctor (even a vet) is going to be extremely important, and in very, very short supply.
And the instinct to survive is not 'human'. It's a basic animal instinct, unlike compassion. Being able to rise above animal instinct is what makes us human.